Composite carbon-coated phosphate material and application thereof

Abstract

The invention discloses a composite carbon-coated phosphate material and application thereof, and belongs to the technical field of battery positive electrode materials. The composite carbon-coated phosphate material is prepared by the following preparation method: mixing manganese salt, a first ligand forming a metal-organic framework and a first solvent for reaction to obtain a Mn-MOF precursor template; s2, mixing the Mn-MOF precursor template obtained in the step S1 with potassium ferricyanide for reaction, and performing ion and ligand exchange reaction to obtain a nitrogen-containing FexMn1-x-MOF template; mixing an iron source, a second ligand forming a metal-organic framework, a second solvent and the nitrogen-containing FexMn1-x-MOF template obtained in the step S2 for reaction; and S3, grinding the product obtained in the step S3, a lithium source and a phosphorus source, performing spray drying, and performing sintering treatment to obtain the composite carbon-coated phosphate material. The material has excellent conductivity, the prepared lithium ion battery has excellent electrochemical performance, and the problem of manganese dissolution can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for batteries, and more specifically, to a composite carbon-coated phosphate material and its application. Background Art

[0002] Due to its advantages of high safety and low cost, lithium iron phosphate has become the mainstream cathode material for lithium-ion batteries in the fields of new energy vehicles and large-scale energy storage today. However, due to its relatively low voltage platform (3.4V), there is limited room for improvement in its performance such as capacity and energy density. Therefore, there is an urgent need to develop new high-energy-density lithium battery cathode materials. Compared with lithium iron phosphate, lithium manganese iron phosphate has a similar olivine-type structure, theoretical specific mass capacity, and excellent kinetic and thermodynamic stability. By doping manganese elements into lithium iron phosphate to form a solid solution material of lithium manganese iron phosphate, it can effectively combine the characteristics of high safety and stability of lithium iron phosphate. At the same time, by introducing manganese elements, the voltage platform of the material can be increased, thereby improving the energy density of the phosphate material, making it a cathode material for lithium-ion batteries with high energy density and high safety. However, as the manganese doping ratio increases, the Jahn-Teller effect and manganese dissolution phenomenon caused by Mn 3+ lead to poor cycle stability of lithium manganese iron phosphate. In addition, due to the low electron and ion conductivity of lithium manganese phosphate, the conductivity of lithium manganese iron phosphate still needs to be improved.

[0003] Metal-organic framework (MOF) is a three-dimensional framework with periodic atomic arrangement, a porous coordination polymer formed by organic bridging molecules (i.e., ligands) and metal ions / metal clusters. Preparing lithium manganese iron phosphate cathode material with metal-organic framework as the precursor can make each metal element evenly distributed; in addition, the organic ligands in the metal-organic framework can serve as effective carbon sources to form a carbon layer on the surface of the material during the sintering process, which can improve the material conductivity and reduce direct contact with the electrolyte on the one hand, and inhibit the excessive growth and aggregation of nanoparticles during the sintering process and relieve the volume expansion effect during the charge and discharge process on the other hand.

[0004] Chinese Patent CN119263244A discloses an in-situ carbon-coated cobalt-doped lithium iron manganese phosphate and its preparation method, which includes the following steps: S1. Dissolve an iron source, a manganese source, and a cobalt source in a solvent to obtain solution A; S2. Mix an organic ligand for forming a metal-organic framework with the solvent evenly to obtain solution B; S3. Add solution A to solution B, stir evenly, react in an inert atmosphere to obtain a mixed slurry, then perform solid-liquid separation and washing to obtain a metal-organic framework precursor; S4. Mix the metal-organic framework precursor obtained in step S3 with a lithium source and a phosphorus source, perform grinding treatment and spray drying, and perform sintering treatment in an inert atmosphere to obtain in-situ carbon-coated cobalt-doped lithium iron manganese phosphate. Its Co element is evenly distributed in the lattice of lithium iron manganese phosphate and forms site doping, which is beneficial to inhibiting the dissolution of manganese and effectively improving the conductivity of the material, thereby enhancing the electrochemical performance of lithium iron manganese phosphate as a cathode material. In addition, using Fe x Mn 1-x-y Co y -MOF as a carbon source template, a uniformly coated carbon layer can be in-situ generated on the surface of the cobalt-doped lithium iron manganese phosphate material through high-temperature sintering without adding an external carbon source, improving the conductivity of the material and alleviating the volume expansion effect during charge and discharge. However, Co doping causes the easy formation of non-electrochemically active phases or cobalt-containing phosphate solid solution phases in the lithium iron manganese phosphate cathode material, affecting the actual capacity performance and further reducing the electrochemical performance of the composite material. Moreover, the conductivity, manganese dissolution, and electrochemical performance problems of this lithium iron manganese phosphate cathode material still need to be further improved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects and deficiencies that the electrochemical performance of lithium-ion batteries prepared by the existing carbon-coated lithium iron manganese phosphate needs to be further improved, and to provide a composite carbon-coated phosphate material with excellent conductivity. The prepared lithium-ion battery has excellent electrochemical performance and can also solve the problem of manganese dissolution.

[0006] Another object of the present invention is to provide a positive electrode sheet.

[0007] Another object of the present invention is to provide a lithium-ion battery.

[0008] The above objects of the present invention are achieved by the following technical solutions:

[0009] A composite carbon-coated phosphate material is prepared by the following preparation method:

[0010] S1. Mix a manganese salt, a first ligand for forming a metal-organic framework, and a first solvent and react to obtain a Mn-MOF precursor template;

[0011] S2. Mix the Mn-MOF precursor template obtained in step S1 with potassium ferricyanide and react to obtain a nitrogen-containing Fe x Mn 1-x -MOF template;

[0012] S3. Mix and react an iron source, a second ligand for forming a metal-organic framework, a second solvent, and the nitrogen-containing Fe x Mn 1-x -MOF template;

[0013] S4. Preparation of the composite carbon-coated phosphate material: Grind, spray-dry, and sinter the product obtained in step S3, a lithium source, and a phosphorus source to obtain the composite carbon-coated phosphate material;

[0014] Among them, in the nitrogen-containing Fe x Mn 1-x -MOF, 0 < x < 1;

[0015] The first ligand is a carboxylic acid organic ligand, and the second ligand is a nitrogen-containing organic ligand.

[0016] In the preparation method of the composite carbon-coated phosphate material of the present invention, the precursor template of Mn-MOF is first synthesized, and then mixed and reacted with potassium ferricyanide. The nitrogen-containing Fe x Mn 1-x -MOF is synthesized by an ion and ligand exchange strategy. The obtained nitrogen-containing Fe x Mn 1-x -MOF contains a nitrogen-containing (-CN) ligand. When preparing lithium iron manganese phosphate coated with carbon using it as a template, nitrogen atoms can be uniformly doped into the carbon coating layer. In addition, on the surface of the nitrogen-containing Fe x Mn 1-x -MOF, Fe-MOF coordinated by a nitrogen-containing organic ligand is in-situ grown, and after post-treatment, a carbon-coated lithium iron phosphate coating layer is formed. Then, using the nitrogen-containing Fe x Mn 1-x -MOF / Fe-MOF as the iron, manganese, and carbon sources, and mixing, grinding, granulating, drying, and one-step sintering with the lithium source and the phosphorus source can prepare a high-performance cathode material of the composite carbon-coated phosphate material with nitrogen-doped carbon-coated lithium iron manganese phosphate as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell. Since there is a sufficient nitrogen-doped carbon coating layer between the interfaces of the two phosphate heterojunctions, the electron transfer ability between the phosphate and the carbon layer and between the phosphates can be significantly enhanced.

[0017] The specific mechanism is as follows:

[0018] After preparing the Mn-MOF precursor template in the present invention, it is mixed and reacted with potassium ferricyanide. The iron element in potassium ferricyanide and the metal manganese element in Mn-MOF undergo ion exchange, and the cyano group in potassium ferricyanide exchanges with the ligand in the MOF, so that the prepared nitrogen-containing Fe x Mn 1-x -MOF has nitrogen-containing organic ligands. The unsaturated cyano groups on the surface of the nitrogen-containing Fe x Mn 1-x -MOF can act as bridging ligands to coordinate with the surface Fe-MOF, thereby increasing the interaction force between the heterojunction MOFs. In the preparation of nitrogen-containing Fe x Mn 1-x -MOF by ion / ligand exchange, and then in-situ growing Fe-MOF to form nitrogen-containing Fe x Mn 1-x -MOF / Fe-MOF. Among them, the nitrogen-containing organic ligands can be used as the carbon source for uniformly doping nitrogen atoms into the carbon layer to prepare a highly conductive carbon layer. Nitrogen doping can significantly enhance the electron transfer ability between the phosphate and the carbon layer and between the phosphates, and at the same time construct an appropriate amount of defect sites, so that the material can provide more lithium-ion adsorption sites and rich active sites, greatly improving the conductivity and lithium-ion migration kinetics of the material. At the same time, as a uniform coating layer, the nitrogen-doped carbon coating layer can effectively alleviate the volume expansion effect of lithium iron phosphate and lithium manganese iron phosphate, improve the structural stability of the material, and avoid side reactions caused by direct contact between lithium iron phosphate and lithium manganese iron phosphate and the electrolyte, which affect the electrochemical performance of the material. In addition, nitrogen atom doping can regulate the Π conjugate system in the carbon material and at the same time regulate its physical and chemical properties. Electron-rich nitrogen atoms can change the wettability of the material, making the material have excellent electrolyte wettability, thereby optimizing the subsequent processing electrolyte injection process, reducing the processing cost and increasing the energy density of the battery.

[0019] The present invention uses nitrogen-containing Fe x Mn 1-x -MOF / Fe-MOF as a template, and cleverly utilizes the advantages of the periodic uniform distribution, controllable morphology, appropriate specific surface area and porosity, and adjustable composition of MOF (metal-organic framework) atoms, laying a foundation for the synthesis of high-performance lithium iron phosphate and lithium manganese iron phosphate materials.

[0020] Compared with the conventional physical mixing method of mixing carbon source with various raw materials, grinding, spray drying, and sintering, the materials prepared with MOF as a template can make the carbon layer uniformly and densely coat on the surface of lithium iron phosphate / lithium manganese iron phosphate, effectively improving the problems of partial direct contact between some phosphates and the electrolyte and a large amount of free carbon floating on the surface of the material caused by uneven coating in the traditional carbon coating strategy, and effectively improving the conductivity and electrochemical performance of the material.

[0021] In the present invention, a lithium iron phosphate coated with nitrogen-doped carbon shell is uniformly coated on the surface of the lithium iron manganese phosphate core coated with nitrogen-doped carbon, which can effectively inhibit the side reactions caused by the direct contact between the lithium iron manganese phosphate and the electrolyte, alleviate the Jahn-Teller effect and the problem of manganese dissolution caused by Mn 3+ and thus improve the structural stability and electrochemical cycle stability of the lithium iron manganese phosphate material, making the prepared phosphate cathode material have both the high capacity of lithium iron phosphate and the high energy density of lithium iron manganese phosphate.

[0022] Preferably, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is (3-10):1.

[0023] In the present invention, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 can be 3, 4, 5, 6, 7, 8, 9 or 10.

[0024] Preferably, the ratio of the molar amount of potassium ferricyanide in S2 to the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 is (0.05-0.95):1.

[0025] The ratio of the molar amount of potassium ferricyanide in S2 to the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 will directly affect the molar ratio of manganese and iron in the subsequent prepared lithium iron manganese phosphate.

[0026] Preferably, in step S2, the concentration of potassium ferricyanide is 0.01-1 mol.

[0027] Preferably, in step S2, the temperature of the ion and ligand exchange reaction is 15-35 °C, and the time of the ion and ligand exchange reaction is 2-12 h.

[0028] Preferably, in step S3, the temperature of the mixing reaction is 80-150 °C, and the time of the mixing reaction is 6-20 h.

[0029] In some embodiments of the present invention, the first ligand for forming the metal-organic framework can be any carboxylic acid organic ligand well-known in the art.

[0030] Preferably, the first ligand is one or more of phthalic acid, isophthalic acid, terephthalic acid, trimesic acid, pyromellitic acid, 4-hydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2-hydroxyterephthalic acid, 2,3-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid, 2,3-dihydroxyterephthalic acid, 2,5-dihydroxyterephthalic acid, p-hydroxybiphenylcarboxylic acid, malonic acid, succinic acid, glutaric acid, adipic acid.

[0031] In some embodiments of the present invention, the second ligand for forming the metal-organic framework can be any nitrogen-containing organic ligand well-known in the art.

[0032] Preferably, the second ligand is one or more of imidazole ligands, porphyrin ligands, pyridine ligands, pyrazine ligands, azide ligands, amino ligands, and nitrogen-containing carboxyl ligands.

[0033] More preferably, the second ligand is one or more of 2-aminoterephthalic acid, 4-aminobenzoic acid, 5-aminoisophthalic acid, imidazole, 2-methylimidazole, 2-aminoimidazole, triazole, benzimidazole, 2-methylbenzimidazole, 5-methylbenzimidazole, 5,6-dimethylbenzimidazole, pyridine, p-aminopyridine, 2-aminopyridine, 3-aminopyridine, 1,2,4-triazole, 1,2,3-triazole, 2,2'-bipyridine-5,5'-dicarboxylic acid, tetracarboxyphenylporphyrin, 4,4'-bipyridine-2,2'-dicarboxylic acid, 5-nitroisophthalic acid, nitroterephthalic acid, dicyandiamide, melamine, and urea.

[0034] Preferably, the first solvent and / or the second solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, methanol, ethanol, isopropanol, and water.

[0035] Preferably, in step S4, the sintering temperature is 700-800 °C and the sintering time is 8-10 h.

[0036] Preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium silicate, lithium sulfate, lithium phosphate, lithium orthosilicate, lithium permanganate, lithium metaphosphate, lithium fluoride, lithium bromide, lithium oxide, lithium nitride, lithium sulfide, lithium oxalate, lithium formate, lithium octanoate, lithium citrate, lithium salicylate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium pyruvate, lithium acetate, lithium methoxide, and lithium ethoxide.

[0037] Preferably, the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, lithium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate.

[0038] Preferably, the iron source is one or more of ferric chloride, ferrous chloride, ferrous sulfate, ferric sulfate, ferric nitrate, ferric acetate, ammonium ferric sulfate, ferric citrate, and ferrous oxalate.

[0039] Preferably, the manganese source is one or more of manganese chloride, manganese sulfate, manganese nitrate, manganese carbonate, manganese acetate, and manganese oxalate.

[0040] Preferably, in step S4, the product obtained in step S3, a lithium source, a phosphorus source, and a dopant are ground, spray-dried, and sintered to obtain a composite carbon-coated phosphate material.

[0041] Preferably, the dopant can be one or more of a cobalt dopant, a magnesium dopant, or a titanium dopant. The cobalt dopant can be cobalt acetate, the magnesium dopant can be magnesium oxide, and the titanium dopant can be titanium dioxide. Based on the mass of lithium iron phosphate / manganese iron phosphate, the doping amount of Co element in the cobalt dopant can be 100-300 ppm, the doping amount of Mg element in the magnesium dopant can be 1500-2500 ppm, and the doping amount of Ti element in the titanium dopant can be 100-300 ppm.

[0042] The present invention also protects a positive electrode sheet, which comprises the above-mentioned composite carbon-coated phosphate material.

[0043] Furthermore, the positive electrode sheet further includes a conductive current collector, a binder, a conductive agent, etc.

[0044] In some embodiments of the present invention, the conductive current collector can be any conductive current collector for a positive electrode known in the art, such as aluminum foil or carbon-coated aluminum foil; the binder can be any binder known in the art, such as one or more of sodium carboxymethyl cellulose, polyvinylidene fluoride, and styrene-butadiene rubber; the conductive agent can be any conductive agent known in the art, such as one or more of acetylene black, graphene, and carbon nanotubes.

[0045] The present invention also protects a lithium-ion battery, which comprises the above-mentioned positive electrode sheet.

[0046] Furthermore, the battery further includes a negative electrode sheet, a separator located between the positive electrode sheet and the negative electrode sheet, and an electrolyte.

[0047] Compared with the prior art, the beneficial effects of the present invention are:

[0048] The present invention discloses a composite carbon-coated phosphate material. By first synthesizing a precursor template of Mn-MOF, and then carrying out the synthesis of nitrogen-containing Fe x Mn 1-x -MOF by an ion and ligand exchange strategy, the obtained nitrogen-containing Fe x Mn 1-x -MOF contains nitrogen-containing (-CN) ligands, and when preparing carbon-coated lithium iron phosphate / manganese iron phosphate as a template, nitrogen atoms can be uniformly doped into the carbon coating layer. In addition, in the nitrogen-containing Fe x Mn 1-x-The Fe-MOF formed by in-situ growth of nitrogen-containing organic ligands on the surface of MOF and coordinated to form a carbon-coated lithium iron phosphate coating layer through post-treatment. Finally, a composite carbon-coated phosphate material with nitrogen-doped carbon-coated lithium manganese iron phosphate as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell is prepared.

[0049] The composite carbon-coated phosphate material of the present invention has excellent conductivity. The prepared lithium-ion battery has excellent electrochemical performance and can also improve the problem of manganese dissolution. Description of the Drawings

[0050] Figure 1 It is a scanning electron microscope characterization diagram of the material in Example 1. Detailed Description of the Invention

[0051] The present invention will be further described below in conjunction with the specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the raw material reagents used in the embodiments of the present invention are conventional purchased raw material reagents.

[0052] Example 1

[0053] A composite carbon-coated phosphate material is prepared by the following preparation method:

[0054] S1. Take 2 mol of 1,4-benzenedicarboxylic acid and 4 mol of manganese chloride tetrahydrate and disperse them in 32 L of a mixed solution composed of N,N-dimethylacetamide and deionized water (N,N-dimethylacetamide: deionized water = 1:1 Vol%). After stirring evenly for 30 min to obtain a dispersion, transfer the dispersion to a reaction kettle and react at 150 °C for 10 h. After cooling to room temperature, wash it 3 times with deionized water and anhydrous ethanol respectively and place it in a vacuum oven at 50 °C to dry overnight to obtain Mn-MOF.

[0055] S2. Take the above Mn-MOF and disperse it in 32 L of anhydrous ethanol and stir for 30 min to form a Mn-MOF dispersion. Take 1 mol of potassium ferricyanide (K3[Fe(CN)6]) and disperse it in 32 L of deionized water, stir for 30 min, then add the aforementioned Mn-MOF dispersion, and stir at room temperature for 6 h. Finally, wash it 3 times with anhydrous ethanol and place it in a vacuum oven at 50 °C to dry overnight to obtain FeMn-MOF.

[0056] S3. Disperse 1 mol of 2-aminoterephthalic acid and 1 mol of ferrous chloride tetrahydrate in 60 L of a mixed solution composed of N,N-dimethylformamide, absolute ethanol, and deionized water (N,N-dimethylformamide: absolute ethanol: deionized water = 8:1:1 Vol%). After stirring for 30 min until homogeneous, add the prepared FeMn-MOF above and stir for another 30 min. Then transfer the mixed dispersion to a reaction kettle and react at 120 °C for 10 h. After cooling to room temperature, wash it three times with deionized water and absolute ethanol respectively, and place it in a vacuum oven to dry overnight at 50 °C to obtain FeMn-MOF / Fe-MOF.

[0057] Among them, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is 5:1.

[0058] S4. Weigh the corresponding lithium carbonate, FeMn-MOF / Fe-MOF, and lithium dihydrogen phosphate according to the stoichiometric ratio of Li: (Mn + Fe): P elements being 1.02:1:1.02, disperse them in ultrapure water, and add cobalt acetate and magnesium oxide dopants based on the mass of lithium manganese iron phosphate / lithium iron phosphate at a doping dose of 200 ppm for Co and 2000 ppm for Mg. Then prepare a slurry with a solid content of 40% for wet grinding and spray granulation drying to obtain the lithium manganese iron phosphate / lithium iron phosphate precursor. Subsequently, place the lithium manganese iron phosphate / lithium iron phosphate precursor in a nitrogen roller hearth furnace and heat it to 780 °C at a heating rate of 3 °C / min and keep it at a constant temperature for 9 h. After naturally cooling to room temperature, after airflow pulverization and sieving treatment, a composite carbon-coated phosphate material with nitrogen-doped carbon-coated lithium manganese iron phosphate LiMn 0.8 Fe 0.2 PO4 as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell is obtained.

[0059] A positive electrode sheet: Mix the above lithium manganese iron phosphate composite material, polyvinylidene fluoride, and acetylene black in a mass ratio of 92:4:4, add N-methylpyrrolidone according to a solid content of 30%, then homogenize, and then coat, dry, and roll to obtain the positive electrode sheet.

[0060] A lithium-ion battery: Use a polypropylene film as the separator, a lithium sheet as the negative electrode, the prepared composite carbon-coated phosphate material positive electrode material as the positive electrode, and 1 mol / L LiPF6 (diethyl carbonate: ethylene carbonate = 1:1 Vol%) as the electrolyte. Assemble the positive electrode, separator, and negative electrode in sequence to obtain the lithium-ion battery.

[0061] Example 2

[0062] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: in step S2, 1.34 mol of K3[Fe(CN)6] is dispersed in 32 L of deionized water, stirred for 30 min, and then the above-mentioned Mn-MOF dispersion is added.

[0063] Obtain lithium manganese iron phosphate LiMn 0.75 Fe 0.25 PO4 as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell to form a composite carbon-coated phosphate material.

[0064] Among them, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is 5.34:1.

[0065] Example 3

[0066] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: in step S2, 2.67 mol of K3[Fe(CN)6] is dispersed in 32 L of deionized water, stirred for 30 min, and then the above-mentioned Mn-MOF dispersion is added.

[0067] S4. Based on the mass of lithium manganese iron phosphate / lithium iron phosphate, doping agents of titanium dioxide and magnesium oxide are added in doses of 200 ppm of Ti doping amount and 2000 ppm of Mg doping amount.

[0068] Obtain lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO4 as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell to form a composite carbon-coated phosphate material.

[0069] Among them, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is 6.67:1.

[0070] Example 4

[0071] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: in step S3, 0.5 mol of 2-aminoterephthalic acid and 0.5 mol of ferrous chloride tetrahydrate are dispersed in 60 L of a mixed solution composed of N,N-dimethylformamide, absolute ethanol and deionized water (N,N-dimethylformamide: absolute ethanol: deionized water = 8:1:1 Vol%), stirred evenly for 30 min, then the above-prepared FeMn-MOF is added and stirred for another 30 min, and then the mixed dispersion is transferred to a reaction kettle and reacted at 120 °C for 10 h. After cooling to room temperature, it is washed 3 times with deionized water and absolute ethanol respectively and placed in a vacuum oven at 50 °C to dry overnight to obtain FeMn-MOF / Fe-MOF.

[0072] To obtain a composite carbon-coated phosphate material with lithium iron manganese phosphate LiMn 0.8 Fe 0.2 PO4 as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell.

[0073] Among them, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is 10:1.

[0074] Example 5

[0075] A preparation method of a composite carbon-coated phosphate material, which is different from Example 1 in that: in step S3, 1.67 mol of 2-aminoterephthalic acid and 1.67 mol of ferrous chloride tetrahydrate are dispersed in 60 L of a mixed solution composed of N,N-dimethylformamide, absolute ethanol and deionized water (N,N-dimethylformamide: absolute ethanol: deionized water = 8:1:1 Vol%), after stirring evenly for 30 min, the prepared FeMn-MOF is added and stirred for another 30 min, then the mixed dispersion is transferred to a reaction kettle and reacted at 120 °C for 10 h. After cooling to room temperature, it is washed 3 times with deionized water and absolute ethanol respectively and placed in a vacuum oven for drying overnight at 50 °C to obtain FeMn-MOF / Fe-MOF.

[0076] To obtain a composite carbon-coated phosphate material with lithium iron manganese phosphate LiMn 0.8 Fe 0.2 PO4 as the core and nitrogen-doped carbon-coated lithium iron phosphate as the shell.

[0077] Among them, the ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is 3:1.

[0078] Example 6

[0079] A preparation method of a composite carbon-coated phosphate material, which is different from Example 1 in that: in step S1, 2 mol of 1,3-benzenedicarboxylic acid is used to replace 2 mol of 1,4-benzenedicarboxylic acid.

[0080] Example 7

[0081] A preparation method of a composite carbon-coated phosphate material, which is different from Example 1 in that: in step S1, 1.34 mol of 1,3,5-benzenetricarboxylic acid is used to replace 2 mol of 1,4-benzenedicarboxylic acid.

[0082] Example 8

[0083] A preparation method of a composite carbon-coated phosphate material, which is different from Example 1 in that: in step S3, 1 mol of 5-aminoisophthalic acid is used to replace 1 mol of 2-aminoterephthalic acid.

[0084] Example 9

[0085] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: in step S3, 2 mol of 4-aminobenzoic acid is used to replace 1 mol of 2-aminoterephthalic acid.

[0086] Comparative Example 1

[0087] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: step S3 is not included.

[0088] Comparative Example 2

[0089] A preparation method of a composite carbon-coated phosphate material, different from Example 1 in that: step S2 is not included.

[0090] Result Detection

[0091] (1) Perform relevant performance tests on the composite carbon-coated phosphate materials of the above examples and comparative examples:

[0092] Powder resistance test: The powder resistance of the composite carbon-coated phosphate materials of the examples and comparative examples was tested by an ST2742B type powder resistivity tester, and the measured results are shown in Table 1.

[0093] The composite carbon-coated phosphate material prepared in Example 1 was characterized by a scanning electron microscope, and the results are as Figure 1 shown. Figure 1 (A) in is the microscopic morphology diagram of the positive electrode material of the composite carbon-coated phosphate material prepared in Example 1 measured at a magnification of 5000 times, Figure 1 (B) in is the microscopic morphology diagram of the positive electrode material of the composite carbon-coated phosphate material prepared in Example 1 measured at a magnification of 20000 times. It can be seen from Figure 1 that the positive electrode material of the composite carbon-coated phosphate material prepared in Example 1 has a nano-level particle morphology, and a uniform conductive carbon layer is coated on the particle surface.

[0094] (2) The composite carbon-coated phosphate materials of the above examples and comparative examples were prepared into lithium-ion batteries by the same method as in Example 1, and relevant performance tests were carried out on the lithium-ion batteries of the above examples and comparative examples:

[0095] Electrochemical performance test: The test conditions were charge and discharge tests at 0.1C and 1C rates respectively at room temperature of 25°C, and the obtained results are shown in Table 1.

[0096] Manganese dissolution amount test: The inductively coupled plasma test method was used to detect the dissolution amount of Mn in the electrolyte of the lithium-ion batteries of the examples and comparative examples after 100 cycles at a current density of 1C, and the obtained results are shown in Table 1.

[0097] The specific test results of each example and comparative example are shown in Table 1 below.

[0098] Table 1

[0099]

[0100]

[0101] Based on the powder resistance test results, it can be seen that the electrical conductivities of the materials in Examples 1 to 3 are far superior to those of Comparative Examples 1 and 2. This can be attributed to the fact that in Examples 1 to 3, FeMn-MOF was synthesized by an ion and ligand exchange method, and Fe-MOF was in-situ grown on the surface of FeMn-MOF, and a composite carbon-coated lithium iron manganese phosphate / lithium iron phosphate cathode material was derived using it as a template. Since the synthesized FeMn-MOF and Fe-MOF materials contain nitrogen-containing organic ligands, they can in-situ form a nitrogen-doped carbon coating layer on the surface of lithium iron manganese phosphate, effectively improving the electronic structure and conductivity of the carbon layer, while reducing the interfacial impedance between the inner core lithium iron manganese phosphate and the outer coating layer lithium iron phosphate, increasing the migration rate of lithium ions and electrons, and thus improving the electronic conductivity and ionic conductivity of the material, and optimizing the discharge capacity, rate performance and cycle stability of the material. Comparative Example 1 is a material with a non-nitrogen-doped carbon-coated lithium iron phosphate shell, and its nitrogen-doped carbon-coated lithium iron manganese phosphate material has relatively poor conductivity due to the doping of manganese atoms in lithium iron phosphate; while Comparative Example 2 is a cathode material composed of a nitrogen-doped carbon-coated lithium manganese phosphate inner core and a nitrogen-doped carbon-coated lithium iron phosphate outer shell. Due to the slow reaction kinetics and extremely poor electronic conductivity of lithium manganese phosphate itself, its powder resistance test results are far worse than those of Examples 1 to 3 and Comparative Example 1.

[0102] From the electrochemical performance test results, it can be seen that the composite carbon-coated lithium iron manganese phosphate / lithium iron phosphate cathode materials prepared from FeMn-MOF / Fe-MOF in Examples 1 to 3, which are composed of an inner core - nitrogen-doped carbon-coated lithium iron manganese phosphate and an outer shell - nitrogen-doped carbon-coated lithium iron phosphate. Due to the unique core-shell structure design and the uniform coating of the high-conductivity carbon coating layer, the discharge capacity, first-cycle charge-discharge efficiency, 1C discharge capacity and 1C cycle performance of Examples 1 to 3 are all superior to those of Comparative Example 1 (non-nitrogen-doped carbon-coated lithium iron phosphate shell) and Comparative Example 2 (composed of a nitrogen-doped carbon-coated lithium manganese phosphate inner core and a nitrogen-doped carbon-coated lithium iron phosphate outer shell). This is because the nitrogen-doped carbon-coated lithium iron manganese phosphate and the nitrogen-doped carbon-coated lithium iron phosphate construct a heterostructure, greatly optimizing the electronic structure and lithium ion migration kinetics of the material. At the same time, the lithium iron manganese phosphate formed by the mutual dissolution of an appropriate amount of lithium manganese phosphate and lithium iron phosphate can effectively increase the energy density of the material, and improve the development bottleneck of the poor conductivity and electrochemical performance of single lithium manganese phosphate.

[0103] From the manganese dissolution test results, it can be seen that compared with Comparative Example 1, due to the composite carbon-coated lithium iron manganese phosphate / lithium iron phosphate cathode material (Examples 1 to 3), the nitrogen-doped carbon-coated lithium iron manganese phosphate core is uniformly coated with nitrogen-doped carbon-coated lithium iron phosphate, which can effectively prevent lithium iron manganese phosphate from directly contacting the electrolyte and inhibit the Jahn-Teller effect and manganese dissolution problem caused by Mn 3+ Furthermore, the structural stability and electrochemical cycle stability of the lithium iron manganese phosphate material are improved. In addition, compared with Comparative Example 2, the substitution of iron atoms for manganese atoms in lithium manganese phosphate fundamentally improves the structural stability of the material and the manganese dissolution problem.

[0104] Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A composite carbon-coated phosphate material, characterized in that, Prepared by the following preparation method: S1. Mix and react a manganese salt, a first ligand for forming a metal-organic framework, and a first solvent to obtain a Mn-MOF precursor template; S2. Mix the Mn-MOF precursor template obtained in step S1 with potassium ferricyanide and react to obtain a nitrogen-containing Fe x Mn 1-x -MOF template; S3. Mix the iron source, the second ligand for forming the metal-organic framework, the second solvent, and the nitrogen-containing Fe x Mn 1-x -MOF template for a reaction; S4. Preparation of the composite carbon-coated phosphate material: Grind, spray-dry, and sinter the product obtained in step S3, a lithium source, and a phosphorus source to obtain the composite carbon-coated phosphate material; Among them, the nitrogen-containing Fe x Mn 1-x -MOF, where 0 < x < 1; The first ligand is a carboxylic acid organic ligand, and the second ligand is a nitrogen-containing organic ligand.

2. The composite carbon-coated phosphate material according to claim 1, wherein The ratio of the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 to the molar amount of the iron source in step S3 is (3 - 10):

1.

3. The composite carbon-coated phosphate material according to claim 1, wherein The ratio of the molar amount of potassium ferricyanide in S2 to the sum of the molar amounts of the manganese salt in S1 and potassium ferricyanide in S2 is (0.05 - 0.95):

1.

4. The composite carbon-coated phosphate material according to claim 1, wherein In step S2, the temperature of the ion and ligand exchange reaction is 15 - 35 °C, and the time of the ion and ligand exchange reaction is 2 - 12 h.

5. The composite carbon-coated phosphate material according to claim 1, wherein In step S3, the temperature of the mixing reaction is 80 - 150 °C, and the time of the mixing reaction is 6 - 20 h.

6. The composite carbon-coated phosphate material according to claim 1, wherein The first ligand is one or more of 1,2-benzenedicarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, 4-hydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2-hydroxyterephthalic acid, 2,3-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid, 2,3-dihydroxyterephthalic acid, 2,5-dihydroxyterephthalic acid, p-hydroxybiphenylcarboxylic acid, malonic acid, succinic acid, glutaric acid, adipic acid.

7. The composite carbon-coated phosphate material according to claim 1, wherein The second ligand is one or more of imidazole ligands, porphyrin ligands, pyridine ligands, pyrazine ligands, azide ligands, amino ligands, nitrogen-containing carboxyl ligands.

8. The composite carbon-coated phosphate material according to claim 7, wherein The second ligand is one or more of 2-aminoterephthalic acid, 4-aminobenzoic acid, 5-aminoisophthalic acid, imidazole, 2-methylimidazole, 2-aminoimidazole, triazole, benzimidazole, 2-methylbenzimidazole, 5-methylbenzimidazole, 5,6-dimethylbenzimidazole, pyridine, p-aminopyridine, 2-aminopyridine, 3-aminopyridine, 1,2,4-triazole, 1,2,3-triazole, 2,2'-bipyridine-5,5'-dicarboxylic acid, tetracarboxyphenylporphyrin, 4,4'-bipyridine-2,2'-dicarboxylic acid, 5-nitroisophthalic acid, nitroterephthalic acid, dicyandiamide, melamine, urea.

9. A positive electrode sheet, characterized in that, The positive electrode sheet contains the composite carbon-coated phosphate material according to any one of claims 1 - 8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery contains the positive electrode sheet according to claim 9.

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